Stabilization of unsaturated nitriles



Patented May 18, 1954 STABILIZATION OF UNSATURATED NITRILE Keith M.Taylor, La Marque, Tex., assignor to Monsanto Chemical Company, St.Louis, Mo., .a corporation of Delaware No Drawing. Application March 11,1953, Serial No. 341,813

12 Claims. 1

This invention relates to the stabilization of unsaturated nitriles.More specifically, this invention relates to the inhibition of thepolymerization of alpha, beta-unsaturated nitriles.

The unsaturated nitriles, particularly the alpha, beta-unsaturatednitriles, are extremely useful chemical compounds. Considerablequantitles of these materials are used for the preparation of syntheticrubbers and various types of synthetic resins. These materials, however,possess a characteristic which all too frequently hampers not only theirutility, but also the purification techniques commonly employed in theirpreparation. All of these materials exhibit a tendency to polymerizewhen subjected to periods of storage and/or to elevated temperatures.Such polymerization results in a darkening of the unsaturated nitrilemonomer and ultimately in the formation of solid polymer contaminant.

Since most applications or uses of the unsaturated nitriles requirehigh-purity products, these polymerization characteristics present aserious problem in the preparation and storage of finished gradematerial. Furthermore, since distillation at elevated temperatures isthe commonly used purification technique, purification is hampered bysolid polymer formation which accumulates in the equipment, ultimatelyplugging the system. To add to this latter problem, impurities presentin crude unsaturated nitriles are frequently of the type whichaccelerate polymerization. For example, acrylonitrile containing smallquantities of acetaldehyde exhibits a greater tendency to polymerizethan does relatively pure acrylonitrile.

Attempts to eliminate this problem have been made in the past by theaddition of inhibitors which retard or prevent the polymerization ofunsaturated nitriles. Unfortunately, the inhibitors heretofore used,while exhibiting some inhibitory effects, have not proven to becompletely satisfactory. To be of practical utility, the inhibitor mustnot impart any undesirable color to the unsaturated nitrile, besufliciently inhibitory to manifest its efiect over extended periods ofstorage, inhibit polymer formation at elevated temperatures to permitutility in distillation purification techniques, and be readilyseparable from the unsaturated nitrile, or inert in subsequent reactionsof the inhibited product. The inhibitors heretofore used have notsatisfied all of these requirements.

It is an object of this invention to provide an improved method forpreventing or inhibiting the polymerization of unsaturated nitriles. It

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is a further object of this invention to provide an improved method forpreventing the polymerization of unsaturated nitriles by the addition ofan inhibitor which does not possess the deficiencies exhibited by theseinhibitors heretofore used. A still further object of this invention isto provide novel stabilized compositions comprising an unsaturatednitrile and an added polymerization inhibitor, which then can besubiected to extended periods of storage or distillation at elevatedtemperatures without exhibiting any significant evidence of polymerformation. Further objects of this invention will become apparent fromthe description which follows.

It has been discovered that the polymerization of unsaturated nitriles,particularly the alpha, beta-unsaturated nitriles, can be successfullyinhibited by mixing therewith a minor amount of the compounds,e-nitroso-p-naphthol. It has been found that a composition comprising anunsaturated nitrile and a minor amount of a-nitroso-p-naphthol will notexhibit any significant polymer formation on prolonged periods ofstorage or when subjected to the temperatures commonly encounteredduring purification by distillation techniques.

The following examples illustrate but do not limit this invention:

Erample I A sealed tube containing 20 ml. of crude acrylonitrilecontaining 85-90% acrylonitrile, 3-7% water, 48% acetaldehyde and smallquantities of lactonitrile, I-I CN, cyanobutadiene, and methyl vinylketone is placed in an oven maintained at 0. The first cloudiness orturbidity m the solution indicative of polymerization was observed aftera period of about 10 hours.

Example [I Emamllle III Samples of acrylonitrile, methacrylonitrile,crotononitrile, u ethylacrylonitrile, p hexylacrylonitrile,a,p-dirnethyl acrylonitrile, e-furfuryl acrylonitrile, p-butylacrylonitrile, p-(2- chloroethyDacrylonitrile, p-ethylacrylonitrile, a-

naphthol acrylonitrile, a-(I-hYdl'OXY phenyl) acrylonitrile arestabilized by mixing therewith about 100 parts'by weight ofa-nitroso-p-naphthol per million parts of the nitrile. After extendedperiods of storage at atmospheric temperatures, no significantdiscoloration or polymer formation is'evident in these samples.

Example IV A 500-111]. round-bottomed flask equipped with a thermometer,stirrer, and sample entry tube is immersed in a constant temperaturebath maintained at 35 C. In this is placed 150 ml. of water throughwhich nitrogen is bubbled at a slow continuous rate. Then 0.1560 g. ofNaHSOa is added and the mixture is allowed to attain thermalequilibrium. After equilibrium is reached, g. of pure acrylonitrile isadded followed by 0.405 g. of KzSzOa. The mixture of sodium bisulfiteand potassium persulfate serves as a polymerization accelerator.Polymerization is allowed to continue for one hour following the end ofthe induction period indicated by a slight turbidity developing in themixture. After the hour has elapsed, the polymer formed is removed byfiltration, washed with water and acetone and dried at TI-83 C.Approximately 7.3 g. of polymer is obtained, indicating a polymer yieldof approximately 73% based on acrylonitrile charged.

Example V The procedure set forth in Example IV is repeated with theexception that 0.060 g. of anitroso-p-naphthol is added to the 150 ml.of water initially placed in the reaction vessel. In this case, nopolymer was formed in the system.

Example VI Crude acrylonitrile of the approximate composition given inExample I is fed at a rate of 200 ml./hr. into a packed column having-20 theoretical plates and operated at atmospheric pressure with anoverhead temperature of 40 C. and a reboiler temperature of 78 C.Acrylonitrile is drawn oil near the bottom of the column. After 394hours of continuous operation the column was dismantled and the polymerformed in both the top and bottom sections was weighed. Approximately19.4 g. of polymer had been formed in the top and 33.4 g. had beendeposited in the lower sections, respectively.

Example VII The procedure of Example VI is repeated under the sameconditions except that a solution of 0.2% a-nitroso-p-naphthol in crudeacrylonitrile is continuously added at a rate of 10 ml./hr. to thereflux to the column so as to maintain a concentration of the naphtholcompound of 100 parts by weight per million parts of acrylonitrile.After 652 hours of operation, the amount of polymer formed wasdetermined as in Example VI by weighing. In this distillation whereu-Ilitroso-s-naphthol was used as an inhibitor, no polymer was formed inthe top of the column and only 3.1 g. of polymer were found in thebottom of the column.

While the preceding examples have illustrated specific embodiments ofthis invention, obviously substantial variation is possible withoutdeparting from the scope thereof. For example, the quantity ofa-nitroso-p-naphthol used to inhibit the polymerization of theunsaturated nitrile can be varied widely, depending upon the particularnitrile to be stabilized and the degree of inhibition desired. Ingeneral, minor amounts of anitroso-p-naphthol are used. An advantage ofa-nitroso-fl-naphthol in such an application as this is the fact that itcan be used in extremely small quantities due to its strong inhibitoryqualities. Concentration in the range of from about 0.5 at about 1,000parts by weight of anitroso-fl-naphthol per million parts of the nitrileare highly preferred. These low concentrations afford adequate stabilityfor most applications and greatly simplify subsequent processing of thenitrile. Of course, higher concentrations of the inhibitor in thenitrile can be used if desired, resulting in a nitrile monomer even morestrongly inhibited against polymerization.

a-Nitroso-s-naphthol can be used to stabilize unsaturated nitriles underany types of conditions. Thus, they can'be used to inhibit polymerformation in unsaturated nitriles during storage under atmospheric.conditions of light, air, temperature and pressure and duringpurification of the crude nitrile monomer. To inhibit polymer formationduring distillation of the unsaturated nitrile, the inhibitor can beadded to the feed stream to the column or introduced separately at somepoint in the distillation column. Preferably, the inhibitor isintroduced at the top of the column to provide maximum inhibitionthroughout the entire system.

The alpha, beta-unsaturated nitriles, represented by the formula whereinthe Rs are the same or different and selected from the group consistingof hydrogen and alkyl radicals containing from 1 to 8 carbon atoms,respond readily to the stabilizing effect of the inhibitors disclosedherein. Acrylonitrile, alpha-substituted acrylonitriles wherein thesubstituent on the alpha carbon atom is an alkyl radical containing from1 to 8 carbon atoms, and beta-substituted acrylonitriles wherein thesubstituent on the beta carbon atom is an alkyl radical containing from1 to 8 carbon atoms represent a class of monomers which polymerize veryreadily, but can be most effectively inhibited against polymerization bymeans of a-IlltlOSO- p-naphthol.

When desired, separation of a-nitroso-p-naphthol from the stabilizedunsaturated nitrile monomer can be accomplished by any of a variety ofsimple techniques. For example, it can be extracted by means of asuitable solvent. Preferably, however, the unsaturated nitrile isdistilled from the inhibited composition.

What is claimed is:

1. A composition comprising an unsaturated nitrile containing a minoramount of a-nitrosop-naphthol.

2. -A composition comprising an alpha, betaunsaturated nitrilecontaining a minor amount of a-nitroso-p-naphthol.

3. A composition comprising a nitrile selected from the group consistingof acrylonitrile, the alpha-substituted acrylonitriles wherein thesubstituent on the alpha carbon atom is an alkyl radical containing from1 to 8 carbon atoms, and the beta-substituted acrylonitriles wherein thesubstituent on the beta carbon atom. is an alkyl radical containing from1 to 8 carbon atoms, containing a minor amount of oznitroso-g-naphthol.

4. A composition comprising a nitrile selected from the group consistingof acrylonitrile, the

alphaepsubstituted acrylonitriles wherein the substituent on the alphacarbon atom is an alkyl radical containing from 1 to 8 carbon atoms, andthe beta-substituted acrylonit'riles wherein the substituent on the betacarbon atom is an alkyl radical containing from 1 to 8 carbon atoms,containing a-nitroso-fi-napthol' in the amount of from about 0.5 toabout 1,000 parts by weight er million parts by weight of nitrile.

5. A composition comprising acrylonitrile and from about 0.5 to about1,000 parts by weight of a-nitroso-B-naphthol per million parts ofacrylonitrile.

6. A process for inhibiting the polymerization of an unsaturated nitrilewhich comprises mixing therewith a minor amount of oc-Ilitl'OSO-B-naphthol.

7. A process for inhibiting the polymerization of alpha,beta-unsaturated nitriles which comprises mixing therewith a minoramount of anitroso-fi-naphthol.

8. A process for inhibiting the polymerization of a nitrile selectedfrom the group consisting of acrylonitrile, the alpha-substitutedacrylonitriles wherein the substituent on the alpha carbon atom is analkyl radical containing from 1 to 8 carbon atoms, and thebeta-substituted acrylonitriles wherein the substituent on the betacarbon atom is an alkyl radical containing from 1 to 8 carbon atoms,which comprises mixing therewith a minor amount of a-nitrosofi-naphthol.

9. A process for inhibiting the polymerization of a nitrile selectedfrom the group consisting of acrylonitrile, the alpha-substitutedacryloni triles wherein the substituent on the alpha caramount of fromabout 0.5 to about 1,000 parts by weight per million parts by weight ofnitrile. 10. A process for inhibiting the polymerization ofacrylonitrile which comprises mixing therewith from about 0.5 to about1,000 parts by weight of a-nit1'oso,8-naphthol per million parts ofacrylonitrile.

11. A process for inhibiting the polymerization of a nitrile selectedfrom the group consisting of acrylonitrile, the alpha-substitutedacrylonitriles wherein the substituent on the alpha carbon atom is analkyl radical containing from 1 to 8 carbon atoms, and thebeta-substituted acrylonitriles wherein the substituent on the betacarbon atom is an alkyl radical containing from 1 to 8 carbon atoms,during distillation which comprises distilling the nitrile in thepresence of a minor amount of a-nitroso-fi-naphthol.

12. A process for inhibiting the polymerization of acrylonitrile duringdistillation which comprises distilling acrylonitrile in the presence ofa minor amount of a-nitroso-s-naphthol.

No references cited.

1. A COMPOSITION COMPRISING AN UNSATURATED NITRILE CONTAINING A MINORAMOUNT OF A-NITROSOB-NAPHTHOL.